Sex would never have evolved unless it offered some advantage. And, the advantage had to be fairly short-term, because living creatures can't wait. So, science needs to explain what that advantage was.
The long term advantage is fairly clear. Sex causes a mix-and-match of slightly dissimilar genetic information. Children differ from each other, as you can see in the world today. When times are uncertain, having all the children identical would be a bad idea. And in fact there are single celled creatures today which reproduce asexually when times are good, and reproduce sexually when trouble looms.
It follows that creatures using sex can evolve faster. This is great, but again, it's long term. We must find a short term gain.
One recent theory is that parasites (or predators or diseases) were a major problem. A species with a lot of variation is much less susceptible to any one kind of attack. Also, the attackers have to be less specialized (and hence less efficient) if they want to find enough susceptible victims. If your competition does not have variation, then one really good parasite could decimate them, leaving the field to you.
There is also a chromosome damage theory. A haploid cell only has one copy of each piece of genetic information. Therefore, any mutation is liable to impair some useful function. A diploid cell has two copies, so if one copy gets damaged, the other copy will still work. The function it controls will still get done, and the cell is quite likely to survive the damage.
This theory says that eukaryotic cells needed much more genetic material than a primitive cell had needed. The more material, the more chances for damage. So, eukaryotes had more need of a damage survival technique.